When an imaging target is imaged through a video camera under illumination with a fluorescent lamp operated by a commercial AC power supply (home power supply), a video signal as the imaging output involves time-dependent brightness variation, i.e., a so-called fluorescent flicker depending on the difference between the frequency of luminance variation (light amount variation) of the fluorescent lamp (this frequency is twice the power supply frequency) and the vertical synchronous frequency of the camera (imaging frequency).
For example, when an imaging target is imaged by an NTSC (vertical synchronous frequency (field frequency, in this case) is 60 Hz) CCD camera in a local area in which the commercial AC power supply frequency is 50 Hz and under illumination with a non-inverter-type fluorescent lamp (although not limited to the case of using a non-inverter-type fluorescent lamp because an inverter-type fluorescent lamp also involves a flicker if its rectification is insufficient), as shown in FIG. 10, the time period of one field is 1/60 seconds while the cycle of luminance variation of the fluorescent lamp is 1/100 seconds. Therefore, the exposure timings of the respective fields involve offsets with respect to the luminance variation of the fluorescent lamp, and hence the exposure amounts of the respective pixels change field by field.
Accordingly, when the exposure time is 1/60 seconds for example, the exposure amount is different among periods a1, a2, and a3 although these periods have the same exposure time. Also when the exposure time is shorter than 1/60 seconds (and when it is not 1/100 seconds to be described later), the exposure amount is different among periods b1, b2, and b3 although these periods have the same exposure time.
The relationship between the exposure timing and the phase of the luminance variation of the fluorescent lamp reverts to the initial state at every three fields, and hence the brightness variation due to the flicker has a repetition cycle of three fields. That is, the luminance ratio among the respective fields (how the flicker looks) changes depending on the exposure time, while the frequency of the flicker is the same irrespective of the exposure time.
When a progressive scan camera such as a digital still camera is used and thus the vertical synchronous frequency (frame frequency, in this case) is 30 Hz, the same brightness variation is repeated every three frames.
A fluorescent lamp normally employs plural fluorescent substances of e.g. red, green and blue in order to emit white light. However, these fluorescent substances each have a unique afterglow characteristic, and provide attenuated emission based on the respective afterglow characteristics during the periods, existing in the cycle of the luminance variation, from a glowing end to the next glowing start. Therefore, during these periods, light that is initially white light is attenuated with the hue thereof gradually changing. Consequently, the above-described offsets of the exposure timings lead not only to the brightness variation but also to hue variation. Furthermore, a fluorescent lamp has a specific spectroscopic characteristic in which a strong peak exists at a particular wavelength, and thus a variation component of a signal is different from color to color.
The hue variation and difference of a variation component on each color basis lead to the occurrence of a so-called color flicker.
On the contrary, when the power supply frequency is 50 Hz and the vertical synchronous frequency of the imaging device is 60 Hz like in FIG. 10, if the exposure time is set to 1/100 seconds, which is the same as the cycle of luminance variation of the fluorescent lamp, as shown in the lowest row in FIG. 10, the exposure amount is constant irrespective of the exposure timing, which causes no flicker.
Actually, a scheme has been devised in which the exposure time is set to 1/100 seconds when the camera is used under illumination with a fluorescent lamp by detecting the condition that the camera is used under illumination with a fluorescent lamp through user's operation or signal processing in the camera. According to this scheme, the occurrence of a flicker can be prevented completely with a simple method.
However, this scheme precludes the exposure time from being optionally set to any time, which reduces the flexibility of the exposure amount adjuster for obtaining adequate exposure.
To address this, another method for reducing a fluorescent flicker without defining the shutter speed has also been devised. In an imaging device in which all the pixels in one image plane are exposed at the same exposure timing, such as a CCD imaging device, only inter-frame brightness variation and inter-frame color variation appear due to a flicker, and therefore reduction in the flicker can be achieved comparatively easily.
For example, in the example of FIG. 10, the repetition cycle of the flicker is three fields unless the exposure time is 1/100 seconds. Therefore, the flicker can be suppressed to a level causing no problem in practical use by predicting the luminance and color variation of the present field from the video signal of the field three fields before the present field so that the average of video signals of the respective fields is kept constant, and then adjusting the gains of the video signals of the respective fields depending on the prediction result.
On the other hand, in an XY-address scanning-type imaging element such as a CMOS imaging element, the exposure timing is sequentially shifted by one cycle of the reading clock (pixel clock) pixel by pixel in the horizontal direction of the image plane, and hence the exposure timings of all the pixels are different from each other. Accordingly, the above-described method cannot suppress the flicker sufficiently.
FIG. 11 shows an operation example of an XY-address scanning-type imaging element. As described above, the exposure timing is sequentially delayed on each pixel basis also in the horizontal direction of the image plane. However, one horizontal cycle is sufficiently shorter than the cycle of luminance variation of a fluorescent lamp. Therefore, the following description is based on an assumption that the exposure timings of the pixels on the same line are identical to each other, and the exposure timings of the respective lines across the vertical direction of the image plane are shown based on this assumption. In an actual case, even such an assumption causes no problem.
As shown in FIG. 11, in a CMOS imaging device as an example of the XY-address scanning-type imaging device, the exposure timing is different from line to line (F0 indicates the difference of the exposure timing among the respective lines in a certain field), and hence a difference in the exposure amount arises among the respective lines. Therefore, not only inter-field brightness variation and inter-field color variation but also in-field brightness variation and in-field color variation occur due to a flicker. On the image plane, this in-field variation appears as a streak pattern (the direction of the streaks themselves is the horizontal direction, and the direction of changes of the streaks is the vertical direction).
FIG. 12 shows an example of the in-field (in-image plane) flicker when an imaging target is based on a uniform pattern. Since one cycle (one wavelength) of the streak pattern is 1/100 seconds, a streak pattern of 1.666 cycles arises in one image plane. If the number of reading-out lines per one field is defined as M, the number of reading-out lines corresponding to one cycle of the streak pattern is L=M*60/100. Note that an asterisk (*) is used as a sign of multiplication in the present specification and drawings.
As shown in FIG. 13, three fields (three image planes) correspond to five cycles (five wavelengths) of the streak pattern, and the streak pattern looks like a pattern flow in the vertical direction when being viewed continuously.
Although only brightness variation due to a flicker is shown in FIGS. 12 and 13, the above-described color variation is also added to the brightness variation actually, which significantly deteriorates the image quality. In particular, the color flicker becomes more prominent as the shutter speed becomes higher (the exposure time becomes shorter). Furthermore, in an XY-address scanning-type imaging device, the influence of the color flicker appears on the image plane, and therefore the image quality deterioration is more noticeable.
Also in the case of such an XY-address scanning-type imaging device, when the power supply frequency is 50 Hz and the vertical synchronous frequency of the imaging device is 60 Hz like in FIG. 11 for example, setting the exposure time to 1/100 seconds, which is the same as the cycle of luminance variation of the fluorescent lamp, allows the exposure amount to be constant irrespective of the exposure timing, which prevents the occurrence of a fluorescent flicker including an in-plane flicker.
However, if the allowable exposure time is limited to 1/100 seconds for prevention of a flicker, the flexibility of the exposure amount adjuster for obtaining adequate exposure is reduced.
To address this, some methods have been proposed to reduce a fluorescent flicker inherent in an XY-address scanning-type imaging device such as a CMOS imaging device without thus defining the shutter speed.
Specifically, Patent Document 1 (Japanese Patent Laid-open No. 2000-350102) and Patent Document 2 (Japanese Patent Laid-open No. 2000-23040) disclose a method in which a flicker component is estimated through measurement of the light amount of a fluorescent lamp by use of a light-receiving element and a light-measuring element, and the gain of a video signal from the imaging element is controlled depending on the estimation result. However, in this method, the addition of the light-receiving element and light-measuring element to the imaging device problematically increases the size and costs of the imaging device system.
Patent Document 3 (Japanese Patent Laid-open No. 2001-16508) discloses another method. In this method, two kinds of images are captured under two conditions: with use of a first electronic shutter value that is suitable for the present external light condition; and with use of a second electronic shutter value that has a predetermined relation to the blinking cycle of the fluorescent lamp, and then a flicker component is estimated by comparing the signals of both the conditions with each other to thereby control the gain of a video signal from the imaging element depending on the estimation result. However, this method is not suitable for capturing of a moving image.
Furthermore, Patent Document 4 (Japanese Patent Laid-open No. 2000-165752) discloses a method in which a correction coefficient is calculated from two video signals obtained through exposure with such a time interval that the phases of flickers of the video signals are shifted from each other by just 180 degrees, so that the video signals are corrected with use of the calculated correction coefficient. However, this method is also not suitable for capturing of a moving image.
In addition, Patent Document 5 (Japanese Patent Laid-open No. Hei 11-164192) discloses another method. In this method, the behavior of brightness variation under illumination with a fluorescent lamp is recorded in a memory as a correction coefficient in advance, and the phase of a flicker component is detected from a video signal from the imaging element by use of the difference of the frequency between the video signal component and flicker component, to thereby correct the video signal with use of the correction coefficient in the memory depending on the detection result.